Electric machines are utilized in a wide variety of applications. For example, hybrid/electric vehicles (HEVs) typically include an electric traction drive system that includes an alternating current (AC) electric motor which is driven by a power converter with a direct current (DC) power source, such as a storage battery. Motor windings of the AC electric motor can be coupled to inverter sub-modules of a power inverter module (PIM). Each inverter sub-module includes a pair of switches that switch in a complementary manner to perform a rapid switching function to convert the DC power to AC power. This AC power drives the AC electric motor, which in turn drives a shaft of HEV's drivetrain. Traditional HEVs implement two three-phase pulse width modulated (PWM) inverter modules and two three-phase AC machines (e.g., AC motors) each being driven by a corresponding one of the three-phase PWM inverter modules that it is coupled to.
Many modern high performance AC motor drives use the principle of field oriented control (FOC) or “vector” control to control operation of the AC electric motor. In particular, vector control is often used in variable frequency drives to control the torque applied to the shaft (and thus finally the speed) of an AC electric motor by controlling the current fed to the AC electric motor. In short, stator phase currents are measured and converted into a corresponding complex space vector. This current vector is then transformed to a coordinate system rotating with the rotor of the AC electric motor.
Recently, researchers have investigated the possibility of using multi-phase machines in various applications including electric vehicles. As used herein, the term “multi-phase” refers to more than three-phases, and can be used to refer to electric machines that have three or more phases. A multi-phase electric machine typically includes a multi-phase PWM inverter module that drives one or more multi-phase AC machine(s). One example of such a multi-phase electric machine is a five-phase AC machine. In a five-phase system, a five-phase PWM inverter module drives one or more five-phase AC machine(s).
In such multi-phase systems, voltage command signals are applied to a pulse width modulation (PWM) module. The PWM module applies PWM waveforms to the voltage command signals to control pulse width modulation of the voltage command signals and generate switching vector signals that are provided to the PWM inverter module. The PWM waveforms are characterized by rising-edge and falling-edge transitions that occur at particular transition angles and define a particular duty cycle during each PWM period. For a pulse train having rectangular pulses, the duty cycle (DC) is the ratio of the pulse-on duration (Ton) and a corresponding PWM period (Tpwm) (i.e., time between the start of consecutive pulses) associated with that same pulse (e.g., for a pulse train in which the pulse duration is 25 microseconds and the PWM period is 100 microseconds, the duty cycle is 0.25 or 25%).
Ideally, these transition angles (i.e., the rising-edge and falling-edge transitions) of a PWM waveform will occur synchronously with respect to the electrical frequency of the machine. The terms “rising-edge” and “leading-edge” are used interchangeably herein. Likewise, the terms “falling-edge” and “trailing-edge” are used interchangeably herein.
However, these transition angles (i.e., the rising-edge and falling-edge transitions) of a PWM waveform occur asynchronously with respect to electrical frequency of the machine. In many operational scenarios (e.g., when the multi-phase machine is operating at between medium to high speed or in an overmodulation region), the transition angles do not always occur at the correct angular locations because the switching frequency of the PWM inverter module is asynchronous with the machine speed. FIG. 1 illustrates an example of switching vector signals (Sa . . . Se) that illustrates three superimposed PWM waveforms where the rising and falling-edges of the duty cycle waveforms do not occur on the correct angular positions during respective PWM periods and are offset from one another and exhibit jitter from cycle to cycle.
Although the PWM module can update/adjust the duty cycle of the PWM waveforms each time it encounters the end “boundary angle” of a current PWM period, this duty cycle is only valid with respect to transition angles that occur before it was calculated, and is not valid for transition angles that occur afterwards.
The PWM module will maintain the duty cycle that was calculated at the end of the current PWM period during the next PWM period that follows (i.e., after the previous duty cycle was calculated). In other words, the PWM module will not adjust the duty cycle of the PWM waveform, but will instead continue to apply the old previously calculated duty cycle even when a transition angle occurs in the next PWM period. For example, when a duty cycle of 100% was calculated at the end of the current PWM period, the processor will not adjust to a duty cycle of 0% when a transition angle takes place in the next PWM period (after which point a duty cycle of 0% is needed). The PWM module will continue to apply the old duty cycle of 100% for the entirety of the next PWM period, and will update the duty cycle (to 0%) only at the end of the next PWM period.
As a result, the wrong phase voltage will be applied to the machine, and therefore, phase current may not be properly regulated, which may in turn cause current/torque oscillations.
It would be desirable to provide a mechanism for ensuring that transition angles occur at their correct angular locations so that the correct phase voltage will be applied to a multi-phase machine to help maintain proper phase current regulation. Other desirable features and characteristics of the present invention will become apparent from the subsequent detailed description and the appended claims, taken in conjunction with the accompanying drawings and the foregoing technical field and background